1. Field of the Invention
This invention relates generally to electrostatic protective devices, and in general to the protection of electrostatic sensitive equipment from static electricity built-up on clean room and other electrostatic protective garments.
2. Description of the Prior Art
The problem of preventing damage to electronic equipment is a serious one. Electrostatic potentials well below the level of human sensitivity (3.5 KV to 4 KV) can cause failure in bipolar and MOS devices, film resistors, silicon chips and many other electronic components, from early processing through assembly to an electronic system's end of life. These potentials can exist on solids or liquids that are electrically isolated from each other and from ground. The procedures and materials traditionally used for electrostatic devise (ESD) protection are in part designed to ensure that these electrostatic potentials are equalized.
Protective containers such as IC rails, bags, and tote boxes--be they static dissipative (10.sup.6 -10.sup.13 ohms /sq) or conductive (less than 10.sup.6 ohms /sq)-operate by allowing the transfer of static electricity across the material to ground. When transported in an ungrounded condition, however, these containers can still develop static potentials. Therefore, before the contents can be safely handled, either mechanically or by a human operator, the container and handler must be at the same potential. Electrostatic charges can be shared by direct connection or via an intervening material, such as a conductive tabletop. In a typical work station designed for ESD protection, the protective container is placed directly on the grounded work surface, while the operator is usually electrically connected to the work surface and to ground with a conductive wrist strap.
Typically, in such surroundings static protective garments are also worn by the persons handling the electronic devices. These protective garments do not have what may be termed a zero static charge but rather produce or generate a minimum amount of static. The static protective characteristics of the fabric are generally achieved by the introduction of carbon yarn or other conductive yarn into the weave of the basic non-conductive fabric, and/or a topical anti-static finish applied to the garment during routine in-use laundry or dry cleaning processes. The basic non-conductive fabric is typically constructed of continuous filament non-conductive polyester. The routine application of a topical anti-stat, depending on its chemical formulation, can either increase the static dissipative properties of a material or minimize its triboelectric charge generating properties by inhibiting its ability to give up or accept electrons, or both. A preferred static protective fabric is one in which conductive fibers are interwoven into the polyester fibers in a grid design (approx. 1/2"). One such fabric is sold under the trademark ULTRA-CHIEF 10 by Worklon, a division of Superior Surgical Mfg. Co., Inc.
Many of the static-protective devices known in the art dissipate an electrical charge generated by a person to a ground; see for example the following U.S. patents:
U.S. Pat. No. 4,398,277 to Christiansen et al; PA1 U.S. Pat. No. 4,373,175 to Mykkanen; PA1 U.S. Pat. No. 3,857,397 to Brosseau; PA1 U.S. Pat. No. 3,596,134 to Burke; PA1 U.S. Pat. No. 3,084,700 to Fischer et al; and PA1 U.S. Pat. No. 1,940,491 to Freitag.
Christiansen et al. describes a device which is strapped onto the users wrist to control the electrostatic charge accumulation on the body of a person. Typically, the wrist strap is a conductive polymer for conducting electrostatic charge via the individuals wrist to a ground cord secured to the wrist strap with an electrically conductive snap connection. The wrist strap can be closed by a Velcro, (trademark of Velcro U.S.A., Inc.) fastener to secure the wrist strap to the wrist of the individual wearer. The strap relies on the firm intimate contact of the conductive member to the body or person of the individual.
Mykkanen describes a similar safety apparatus, e.g., wrist strap which is connected between a human and an electrically conductive terminal.
Brosseau also describes an electrically conducive wrist strap worn on the wrist.
Burke describes an apparatus to be worn by persons working in an electrostatic field for eliminating discomfort and ill effects of discharges of electrical energy through the person. The device comprises one or more bands in the form of garter or a belt, each having a flexible conductive element therein which conforms to the body shape and lies close to the skin. The garter belt is connected by leads to a conductor for contacting grounded structures.
Fischer et al. relates to a grounded comb.
Frietag relates to a device for grounding persons working near high-voltage electric power stations to prevent electrocution of or injury to a person.
Thus, it can be seen that all of the aforedescribed devices are directed solely to discharging electrostatic charge from a person.